1. Field of the Invention
This invention relates to the field of metallization of diamond and various other materials in electronic packaging and other applications wherein thin metallization layers are desired or required, and more particularly wherein high temperature stability of thin metallization layers during further processing and/or device operation is required.
2. Prior Art
In electronic devices and electronic device packaging, metal-diamond contacts are needed for most applications of CVD (chemical vapor deposition) diamond. The requirements on such contacts vary greatly from application to application. Important for many of them are metallurgical stability at elevated temperatures and good adhesion, as for example for the bonding of components on CVD diamond heat sinks for multi-chip modules. These two requirements, however, are difficult to meet with a single film, since good adhesion requires a reaction between the covalently bonded diamond and the metal. The metallization schemes available today for diamond consist of a thin adhesion layer, typically titanium (Ti) or chromium (Cr), that bonds a thicker metal or alloy layer, typically gold (Au) or gold-tin (Au--Sn) to the diamond substrate. Unless the interaction is efficiently prevented, Cr from the adhesion layer will be dissolved in the thicker Au film, since the solubility of Cr in Au is substantial at elevated temperatures. (See Binary Alloy Phase Diagrams 2nd Ed., T. B. Massalski, ASM International, (1990).) The same holds true for Ti in Au, where titanium-gold compounds can form as well. (See Binary Alloy Phase Diagrams 2nd Ed., T. B. Massalski, ASM International, (1990) and J. Vac. Sci. Technol. 9, T. C. Tisone and J. Drobeck, (1972) 271.) These metallurgical interactions degrade the thermal and electrical properties at the bond and decrease the adhesion of the metallization layer, limiting the use of these metallizations to below 200-300.degree. C. The limit for metallizations that use titanium/platinum/gold (Ti/Pt/Au) or titanium-tungsten/gold (Ti--W/Au) metallization schemes to impede diffusion is 450.degree. C. (See ICEMM Proc. '93, H. A. Naseem, I. Meyyappan, C. S. Prasad, W. D. Brown, (1993) 62, and Thin Solid Films 253, I. Meyyappan, A. P. Malshe, H. A. Naseem, W. D. Brown, (1994) 407.) All the commercially available diamond metallization schemes tested by the inventors of the present invention degraded upon vacuum annealing above 450.degree. C.